Given that Days of Future Past is about righting wrongs, I think it makes far more sense that Cyclops and Jean Grey return as a byproduct of altering the past rather than having them survive on the Astral Plane for 10 years. I'm not even sure Cyclops could survive on the Astral Plane, as it's a place typically reserved for powerful psychics, not to mention he doesn't have a body to return to afterward.

Also, the sudden revelation of the Astral Plane as a means of possible survival reminds me too much of the questionable storytelling found in The Last Stand, like the reveal of mutant class levels and the only explanation for Jean's survival being that she wraps herself in a cocoon of telekinetic energy. No thanks.

If the means are already present in Days of Future Past via time travel, there is no need to further complicate things by adding even more outlandish mechanisms to the film.